A hollow fiber membrane is used in a wide range of fields, e.g., a water treatment membrane for producing industrial water or drinking water from seawater, etc., a medical membrane for artificial kidney, plasma separation, etc., a membrane for food-beverage industry such as fruit juice concentration, and a gas separation membrane for separating carbonic acid gas, etc.
As the material of a hollow fiber membrane, in view of, e.g., properties required in respective applications and aptitude for a production method suitable to exhibit the properties, various resin compositions are used. Among others, a cellulose ester has permeation performance due to its hydrophilicity as well as chemical resistance performance of being strong against chlorine bactericides and is therefore widely used as a material of hollow fiber membranes including a water treatment membrane.
For using a cellulose ester as the material to obtain a hollow fiber membrane exhibiting separation performance of removing ions, a phase separation method of solution spinning using a cellulose ester solution containing a large amount of a solvent is known.
For example, Patent Document 1 discloses a hollow fiber membrane having an asymmetric structure, which is produced by mixing N-methyl-2-pyrrolidone, ethylene glycol and benzoic acid with cellulose triacetate, immersing the resulting solution in a solidification bath containing N-methyl-2-pyrrolidone/ethylene glycol/water while discharging it from an arc-type nozzle, and subjecting the obtained membrane to water washing and heat treatment.
Patent Document 2 discloses a composite hollow fiber membrane obtained by forming a thin film that includes a cellulose ester and has an asymmetric structure, on the outer or inner surface of a porous hollow fiber membrane by the phase separation method including immersing the outer or inner side of a porous hollow fiber membrane, such as polyethylene or polysulfone, in a cellulose ester solution containing a large amount of a solvent as well, drying and immersing in water.
On the other hand, an extraction method of obtaining a cellulose ester hollow fiber membrane by simple melt spinning without using a solvent is also known. For example, Patent Document 3 discloses a hollow fiber membrane obtained by mixing a water-soluble polyhydric alcohol having an average molecular weight of 200 to 1,000 with cellulose diacetate, and melt-spinning the resulting solution.
In addition, Patent Document 4 discloses a production method for a composite hollow fiber, in which a mixture of cellulose triacetate, sulfolane and polyethylene glycol as outer layer components, and a mixture of cellulose diacetate and polyethylene glycol as inner layer components are each melted in separate extruders and discharged from a spinneret for two-layer composite hollow fibers.